1. Field of Invention
This invention relates generally to preparing stable dispersions of propylene polymers.
2. The Prior Art
U.S. Pat. No. 3,616,047 teaches sealing the pour opening of easy opening containers with a propylene polymer layered closure member. The closure member is generally a laminate of the propylene polymer resin and a metal foil such as aluminum foil which is heat sealed by its propylene polymer layer to the surface of an end panel coated with a bond promoting layer of a carboxyl modified polypropylene resin. The carboxyl modified polypropylene layer is also used as a bonding layer in the lamination of the propylene polymer to the aluminum foil to prepare the closure member.
Although the propylene polymer closures of U.S. Pat. No. 3,616,047 can be effectively bonded to the end panel to seal the container, problems arose in the application of the carboxyl modified polypropylene layer to the enameled metal surface as well as in preparing the propylene polymer/aluminum foil laminate.
When the carboxyl modified polypropylene resin was utilized as an adhesion promoting layer for laminating the propylene polymer to the aluminum foil layer or to bond the laminate to the metal can end, application of the carboxyl modified polypropylene resin as a coating layer required that the resin be applied as a dispersion in an organic liquid such as kerosene. In order to obtain a dispersion of commercial utility a resin having a particle size of 0.1 to 5 microns was required. At the present time, the resin in such a small particle size range is a relatively expensive material. Further, coating application of the resin as a dilute kerosene dispersion, e.g., 10% solids, was found unsatisfactory as it did not have the required physical properties, such as viscosity and flow out, which are required in conventional coating equipment, e.g., as by roll coating. To obtain a coating dispersion having the physical properties necessary for commercial coating application, polypropylene resin powders of approximately the same particle size range as the carboxyl modified polypropylene, e.g., 0.1-5 microns, were incorporated in the dispersion to raise the solids content to about 20%, whereby the dispersion had the flow and viscosity characteristics required in commercial coating applications. The drawback to the use of polypropylene resins of such particle size was that polypropylene resin of particle sizes less than 5 microns is difficult to manufacture and is presently in limited commercial supply.
U.S. Pat. No. 3,483,276 teaches applying mixtures of polypropylene and carboxyl modified polypropylene as coatings dispersed in xylene. Xylene has a density of 0.86 grams/cubic centimeter (g/cc) and a boiling point of 280.degree.-290.degree. F. When it is attempted to prepare a xylene dispersion of the carboxyl modified polypropylene resin, the dispersion is unstable and the resin separates from the dispersion on standing overnight. When the resin is applied to a metal substrate from a xylene dispersion and then baked to effect removal of the xylene, unsatisfactory flow-out and coalescence of the resin occurs with the result that the coating layer applied to the substrate is rough and lacks integrity, i.e., the coating coverage is incomplete.